Method for manufacturing tube and fin heat exchanger with reduced tube diameter

ABSTRACT

According to a preferred embodiment, an improved method and apparatus for manufacturing tube and fin heat exchangers that includes a step for reducing the outer diameter of stock tubing to a value well below nominal prior to bending the tubing into hairpins. The reduced diameter increases the stiffness of the resultant hairpins and allows for efficient lacing of the heat exchanger fins and end plates. The method uses pre-size rollers that are adjustable for reducing the diameter of the tubing. The invention also includes a hairpin and heat exchanger manufactured according to the method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to tube and fin heat exchangers, and inparticular, to manufacturing processes and equipment for producing tubeand fin heat exchangers, such as for HVAC systems.

2. Description of the Prior Art

As illustrated in FIG. 1, a typical tube and fin heat exchanger (10)consists of a stack of generally planar metallic fins (12) sandwichedbetween a top end plate (14) and a bottom end plate (16). The terms“top” and “bottom” used for designating heat exchanger end plates arederived based on the heat exchanger orientation during expansion in avertical hairpin expander press, as described below. The “top” and“bottom” designations are not necessarily indicative of the heatexchanger orientation in any particular installation.

The fins (12) have a number of collared holes (18) formed therethrough,and the top and bottom end plates (14, 16) have corresponding holes (20)formed therethrough. When the fins (12) and end plates (14, 16) arestacked, the holes (18, 20) are in axial alignment for receiving anumber of U-shaped hairpin tubes (“hairpins”) (22) through the stack.Hairpins (22) are formed by bending lengths of small tubes, typicallycopper, aluminum, steel or titanium, 180 degrees around a small diametermandrel. The hairpin tubes (22) are fed, or laced, through theloosely-stacked assembly of fins from the bottom end plate (16) so thatthe open ends (26) of the hairpin tubes (22) extend beyond the top endplate (14). The top end plate (14) is slipped over the open ends (26) ofthe hairpins (22), and the hairpins (22) are mechanically expanded fromwithin to create a tight fit with the fins (12). Finally, return bendfittings (24) are soldered or brazed to the open ends (26) of thehairpin tubes (22) to create a serpentine fluid circuit through thestack of fins (12).

FIG. 2 is a flow chart diagram that describes the manufacturing processof prior art used to mass produce tube and fin heat exchangers.Referring to both FIGS. 1 and 2, as shown in step (50), fins (12) areformed by a stamping process in a fin press, such as those produced byBurr Oak Tool, Inc. of Sturgis, Mich. Fin stock is delivered to a pressin a roll of sheet metal. Various metals, heat treatments, andthicknesses may be used, but aluminum is the general industry selection.Fin stock is paid out from an uncoiler, lubricated, then fed through apress, where a die draws, details, punches collared holes, and cuts finsto a desired length and width. Stamping generally occurs in severalstages. At the back end of the fin press, fins index out of the dieunder a vacuum hood, where a pressure differential holds them in placeuntil discontinued, at which time (sometimes mechanically assisted bywire) the fins drop from the vacuum hood on to collection rods that passthrough collared holes of the fins. The collection rods are mounted onto collection tables. Once having a full stack of fins, the collectiontable is removed from the back-end of the fin press. Drop rods areinserted into the fin stacks to keep the stacks intact. Operators liftstacks of fins by placing their hand at the bottom of the stack andphysically lift the stack up off the collection table rods. Fin stacksthen are staged for the lacing process as depicted by element (56) inFIG. 2.

As shown by step (52) in FIG. 2, the heat exchanger top and bottom endplates (14, 16) are manufactured in a stamping process that isindependent of the fin stamping process (50). The end plates aretypically made of a fairly stiff sheet metal. The end plates (14, 16)may also each include bends that form a channel or similar profile toprovide strength and rigidity. Holes (20), which align with the collaredholes (18) of the fins (12), are punched through the end plates by apress and die.

The hairpin tubes (22) are manufactured in process step (54). Referringto FIGS. 2 and 3, hairpins are typically formed in a hairpin bendermachine (88), using machines such as vertical bend hairpin bender,manufactured by Burr Oak Tool, Inc. of Sturgis, Mich., to form multiplehairpins at a time. Depending on the outer diameter of the stock tubing,commonly up to six lines of tubing are typically processedsimultaneously in a single hairpin bender machine (88).

A typical vertical bend hairpin bender machine (88) consists of threesections—the tubing pay-out section (90), a feeder section (92), and abender section (94). U.S. Pat. No. 6,354,126, issued to Small, et al.and entitled “Tube Straightener and Drive Therefor,” describes a typicalfeeder section (92), and the patent is incorporated herein in itsentirety by reference. U.S. Pat. No. 5,233,853, issued to Joncs James G.Milliman and entitled “Stretch Straightening Hairpin Bender,” describesa typical bender section, and it is also incorporated herein in itsentirety by reference.

The pay-out section (90) includes a coil stand (96), also known as anuncoiler, for supporting multiple tubing spools or bare-pack tubingcoils. As stock tubing (100) is paid out from bare-pack coils or spoolsat the pay-out section (90), the stock tubing (100) will typicallycontain bends and may be out-of-round at times. Thus, the feeder sectionusually includes correction rollers for reforming the stock tubing backto nominal dimensions.

A more detailed view of the feeder section of a typical hairpin bendermachine is shown in FIG. 4. Cross-axis rollers (102) correct ovality,eccentricity, and out-of-round conditions of the stock tubing (100),returning it to a circular profile. Next, a pair of pre-size rollers(104) typically surrounds and rolls the stock tubing (100) to return anyportion of stock tubing that has a slightly larger than normal outerdiameter to its nominal size. In other prior art hairpin benderconfigurations, a stationary pre-sizing die (not illustrated) is used inplace of the pre-size roller pair (104). A final pair of offsetstraightening rollers (106) ensures the stock tubing (100) is straightand true.

After the trio of correction rollers (102, 104, 106), a pair of conveyorbelts (108) clamps the stock tubing and drives the tubing through thehairpin bender machine (88). Each line of stock tubing (100) beingprocessed by the hairpin bender (88) is fed by the feed belt assembly(108) over a boom (110), a bend arbor clamp (112) and mandrel tip rods(114) in bender section (96). Tube draw for each tubing line continuesuntil that tubing contacts a switch tower. Once all of the tubes havecontacted their respective switch tower, the bend arbor clamp (112)engages and a tube cutter head assembly (98), located at the end of thefeeder section (92), cuts the tubes. Mandrel tips are extended, and boom(110) actuates, bending the cut tube sections 180 degrees about amandrel (115), thus creating the hairpin tubes (22). Once the boom (110)actuates its limit switch (not shown), indicating a complete bend, thebend arbor clamp (112) is released, and a stripper assembly (notillustrated for simplicity) pushes the hairpin tubes (22) out of theboom (110) and off of the mandrel tip rods (114), where the hairpins(22) then fall into catch arms (also not illustrated for simplicity).The hairpins (22) are removed from the catch arms and staged in largeracks for the lacing process as depicted by element (56) in FIG. 2.

As shown in step (56) of FIG. 2, the lacing process is that process inwhich stacks of fins (12), the bottom end plate (16), and the hairpins(22) are assembled together, typically by hand. Fin stacks are laid outon a lacing table, one stack at a time. Drop rods are removed from eachfin stack as multiple stacks are assembled together on the table to forma contiguous slab of fins. The heat exchanger bottom end plate (16) isadded to one end of the slab, and it is temporarily held in place withrods. These rods also help maintain fin alignment until an adequatenumber of the hairpin tubes have been laced through the assembly tomaintain alignment. Hairpins (22) are typically hand-laced through thebottom end plate (16) into the slab of fins (12) one at a time by anoperator who manually finesses them in. For conventional diameterhairpins and fins of the prior art, for example ⅜ inch diameterhairpins, lacing is a simple process, taking on average no more thanfive seconds per hairpin.

At this stage of assembly, the heat exchanger consists of stacks of fins(12) and a bottom end plate (16) loosely held together by hairpins (22)passing transversely through the assembly. As shown in step (58) of FIG.2, in order to form tight metal-to-metal interfaces between the tubesand the fins of the heat exchanger so that efficient conductive heattransfer paths are created between the tubes and the fins, the hairpins(22) are expanded within the fins to create an interference fit.

The laced heat exchanger assembly is placed within a hairpin expandermachine (150), and the top end plate (14) is slipped over the open ends(26) of the hairpins (22). FIG. 6 shows a typical hairpin expander (150)of prior art into which a heat exchanger assembly is vertically placedwith the open ends (26) of the hairpins (22) facing upwards. Referringto FIGS. 1 and 6, hairpin expander (150) has bullets (152) located atthe ends of long rods (154) for passing through the open ends (26) ofthe hairpins (22). Multiple bullets (152) and rods (154), two for eachhairpin (22), are typically provided for simultaneously expanding all ofthe hairpins (22). Each bullet (152) is sized to have an outer diameterlarger than the inner diameter of the hairpin tubes (22). The expanderhas a hydraulic ram (151), that acts upon a pressure plate (153) whichin turn drives rods (154). As the expander (150) presses the bullets(152) into the hairpins (22), the bullets (152) expand the hairpins (22)into a tight, interference-fit engagement with the fins (12).

Expansion of the diameter of the hairpins causes axial shrinkage of thehairpins. Typically, about a 3-5 percent reduction in hairpin lengthoccurs during the prior art expansion process used with conventionalhairpin tube diameters (e.g. ⅜ inch). In the earlier vintage hairpinexpanders (150), the heat exchanger stack is supported in the expanderat the bottom end plate (16) and at the bends (23) of the hairpins (22)by a bolster plate (156). The hairpin bends (23) are supported by areceiver plate or cradle plate (158) that has semicircular grooves (161)cut therein to accommodate them. The receiver or cradle plate (158) isin turn supported on the bolster plate (156). The stack of fins (12) andthe top end plate (14) “float” or rest on the bottom end plate (16). Asthe hairpins are expanded, the hairpins (22) are under compressiveforces. Therefore, expander (150) includes a fixture (160) mounted tothe expander frame, into which the heat exchanger is placed. Fixture(160) includes front and back plates (162) that laterally support thestack of fins (12) to prevent them from buckling during the expansion.Side rails (164) may be included in fixture (160) for making it easierto center the heat exchanger within the expander (150).

Because the top end plate (16) becomes initially fixed in position nearthe tips (26) of the hairpins (22) after expansion is first commenced,there is a concomitant shrinking and tightening of the stack of fins andend plates due to the longitudinal shrinkage of the hairpins (22) duringexpansion. Even with attempts to predict and compensate for theshrinkage of the hairpins with this type of expander, the process stillresults in heat exchangers having large dimensional variances.

The more advanced expanders of prior art employ a coil shrink ratecontrol feature that forces all of the hairpin tubes to shrink at thesame rate. With this type of expander, both the top and bottom endplates (14, 16) are held fixed within the fixture (160) at the desireddimensions, thus providing a finished heat exchanger product of havinghigher dimensional tolerances. The hairpin bends (23) are supported in acradle or receiver plate (158), which is in turn supported by thebolster plate (156). During initial expansion, the hairpins (22) are incompression. However, because the top end plate is held fast within thefixture (160), after the bullets have passed through the top end plate(14), securing the top end plate near the upper ends (26) of thehairpins (22), the compressive hairpin force becomes a tensile force asthe hairpin bends (23) contract and pull away from the cradle orreceiver plate (158). The hairpins (22) are held by the top end plate(14) in the fixture (160) during expansion, and as the hairpins (22)contract in length, the hairpin tubing below the bullets (152) slideupwards within the stack of fins (12), moving the hairpin bends (23)toward the bottom end plate (16).

It is possible that the tensile force exerted on the hairpins (22) atthe top end plate (14) by the bullets (152) during expansion may exceedthe strength of the interference fit that holds the hairpins (22) in theholes (20) of the top end plate (14). If this happens, damage to theheat exchanger will occur. Therefore, with the controlled-shrink-rateexpander, the bolster plate (156), which carries the cradle or receiverplate (158), is designed and arranged to move upwards at the same rateas the hairpin bends (23) move upwards, thus continuing to providesupport of the hairpins.

U.S. Pat. No. 4,780,955 issued to Stroup on Nov. 1, 1988 and entitled“Apparatus for Making a Tube and Fin Heat Exchanger” describes anexpander that employs coil shrink rate control, and the patent isincorporated herein in its entirety by reference. The '955 patentteaches that the bolster plate may be mechanically driven as a functionof the position of the ram cylinder that drives the pressure plate, therods and the expansion bullets. For example, for every inch of downwardtravel of the ram cylinder, a cam arrangement (not illustrated) drivesthe bolster upward 0.03 inches. The '955 patent also discloses a secondarrangement in which a pneumatic actuator (not illustrated) drives thebolster plate upward. The pneumatic actuator force is manually selectedby the operator so as to approximately balance the force applied to thehairpins by the bullets.

In the controlled-shrink-rate expander of prior art described above, theupward movement of the bolster plate and receiver or saddle plate tendsto apply an upward force on the heat exchanger bottom end plate. Thebottom end plate of the heat exchanger is held fast within the expanderfixture by a piano hinge clamp arrangement, as illustrated in FIGS. 7-8.FIG. 7 illustrates the bottom end of a heat exchanger (10) that is to bepositioned within the expander for expanding the hairpins into aninterference-fit engagement within the stack of fins (12). The hairpinbends (23) are shown extending beyond the bottom end plate (16) of thelaced heat exchanger. The heat exchanger fins are not illustrated forsimplicity. Fixture base plate (165) receives and positions the heatexchanger bottom end plate (16) during the shrink-rate-controlledexpansion process described above. Base plate (165) may have a recessedseat (166) into which bottom end plate (16) is received. A slot (167) isformed through base plate (165) for allowing the hairpin bends (23) topass therethrough and to be received into the receiver or cradle plate(158). Two piano hinge clamps (168) (only one is shown for simplicity)are attached to fixture base plate (165) and are arranged to be foldedover the seated bottom end plate (16), retaining the bottom end plate(16) in the recessed seat (166) of the fixture plate (165). A pair oflatch mechanisms (170) are secured to studs (172) in fixture plate(165). The latch mechanisms (170) are rotatable for locking the pianohinge clamps (168) into the folded clamping position or for allowing thepiano hinge clamps (168) to swing freely.

FIG. 8 illustrates the piano hinge clamp arrangement of the fixture baseplate (165) of FIG. 7, with the heat exchanger bottom end plate (16)seated in the recessed seat (166) of the base plate (165). The pianohinge clamps (168) are folded over the heat exchanger bottom end plate(16), holding the end plate fast to fixture base plate (165). Thelatches (170) are rotated to keep the piano hinge clamps (168) in thedownward, folded position.

Referring back to FIGS. 1 and 2, after the expansion process, a numberof short return bend tubes (24) are soldered, brazed or welded to theopen ends (26) of the hairpins (22) to create one or more longserpentine circuits from the hairpins for fluid flow. Additionally, oneor more cross-over tubes (not shown), which connect various hydrauliccircuits, may be soldered, brazed or welded to the open ends (26) of thehairpins (22). In the typical manufacturing process used formanufacturing prior art ⅜ inch tubing heat exchangers, for example, thereturn bends (26) and the cross-over tubes are brazed to the heatexchanger (10) in an autobrazing process, depicted as step (60), in theflowchart of FIG. 2. The tubes are hand-assembled with brazing rings,and the heat exchanger is run through a furnace, wherein the joints arebrazed.

After the autobrazing step (60) in the heat exchanger manufacturingprocess of prior art, a leak check (62) is performed. For each circuit,one end is plugged while a pressure-decay monitoring device is connectedto the other end. If the circuits hold pressure, there are no leaks.

Finally, for heat exchangers used in HVAC systems, in step (64),subcooler, liquid and suction manifolds are manually brazed to the heatexchanger circuits.

There is concern with the effects of R22 refrigerant in depleting theozone layer, and so the new HVAC systems are designed to use R410refrigerant. R410 refrigerant systems operate at higher pressures thantheir R22 counterparts. Higher operating pressure allows the use ofsmaller diameter tubing in heat exchanger coils of condensers and heatpumps. Smaller diameter tubing provides a better ratio of heat transfersurface areas, has merits in terms of pressure drop on the air sidebecause of reduced form drag, and requires less material to provide thesame amount of heat transfer surface area, which is especiallyattractive from a commercial perspective. Consequently, strong desireexists among HVAC manufacturers to design manufacturing processescapable of realizing small diameter product. The current industrystandard diameter is ⅜ inches, although some manufacturers use 7 mm.Other manufacturers use 5 mm coils to produce heat exchangers havingshort lengths, for example no longer than 36 inches. However, when thehairpin tubing becomes too small, both the lacing process and theexpansion process become exceedingly difficult, and commercially viablemanufacturing of any but the smallest heat exchangers has previously notbeen possible. For example, heat exchangers six or more feet in lengthare readily manufactured using ⅜ inch copper tubing. However, when 5 mmcopper tubing is used, before the present invention, it has not beencommercially feasible to manufacture a heat exchanger longer than about36 inches—the 5 mm copper tubing is too flimsy to readily lace andexpand long heat exchangers, and the concomitant manufacturing time istoo long to justify the expense of producing the 5 mm heat exchanger.

It is desirable, therefore, to provide a manufacturing process andsystem that allows tube and fin heat exchangers characterized by smalldiameter hairpin tubing, such as 5 mm copper tubing, to be quickly,easily, and cost-effectively manufactured.

3. Identification of the Objects of the Invention

A primary object of the invention is to provide a manufacturing processthat allows tube and fin heat exchangers of long length to bemanufactured using 5 mm or smaller tubing.

Another object of the invention is to provide pre-lacing hairpin tubingsizing apparatus for use in the above 5 mm manufacturing process.

Another object of the invention is to provide an expander apparatus foruse in the above 5 mm manufacturing process.

Another object of the invention is to provide an apparatus that modifiesexisting tube and fin heat exchanger manufacturing equipment to becapable of manufacturing 5 mm heat exchangers.

Another object of the invention is to provide an improved lacing tablefor use in the above 5 mm manufacturing process.

Another object of the invention is to provide an autobrazing process forthe above 5 mm manufacturing process in which return bends, cross-overs,and all HVAC manifolds are brazed in one step.

Another object of this invention is to provide a pressure test todetermine blockages within the heat exchanger fluid circuits.

SUMMARY OF THE INVENTION

The objects above as well as other features of the invention arerealized in an improved method for manufacturing tube and fin heatexchangers that, according to a preferred embodiment, includes a processfor reducing the outer diameter of stock tubing to a value well belownominal prior to bending the tubing into hairpins. The method usespre-size rollers that are adjusted to reduce the diameter of the tubing.The reduction of diameter increases the stiffness of and imparts anovality to the resultant hairpins that allows for efficient lacing ofthe heat exchanger fins and end plates.

The manufacturing process according to the preferred embodiment alsoincludes expansion of the hairpin tubes with a high expansion ratio—forexample, 8 to 9 percent. During this expansion, the hairpin tube bendsare supported by a programmable, infinitely variable hydraulicallyactuated bolster plate that accurately controls the resultant forcesimparted on the hairpin tubes.

In the preferred embodiment, the manufacturing process further includesa novel autobrazing step in which both return bend fittings andmanifolds are simultaneously brazed to the hairpin tubes using high andlow temperature brazing rings.

The preferred manufacturing process also includes a unique pressure testfor determining tube blockage using a pressure decay tester.

The preferred embodiment of the invention includes an improved hairpinexpander machine equipped with a closed-loop-controlled hydraulicallypositioned bolster plate, a heat exchanger-holding fixture characterizedby a full-length, continuous front plate, back plate, and side rails.Preferably, the expander includes a novel heat exchanger bottom endplate clamping mechanism as part of the fixture.

The preferred embodiment of the invention also includes an improvedlacing table and a hairpin storage bin assembly.

Finally, the invention includes heat exchanges with small diameterhairpin tubing manufactured according to the manufacturing processesdisclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail hereinafter on the basis of theembodiments represented in the accompanying figures, in which:

FIG. 1 is a perspective view of a typical tube and fin heat exchanger ofprior art;

FIG. 2 is a flow chart diagram illustrating the prior art process formanufacturing tube and fin heat exchangers used for HVAC systems;

FIG. 3 is a perspective drawing that illustrates a typical hairpinbender machine of prior art, showing process equipment including anuncoiler, a series of correction rollers, a feed belt drive mechanism, atube cutter and a tube bender;

FIG. 4 is a larger scale perspective drawing of the feed section of thehairpin bender machine of FIG. 3, showing the cross-axis correctionalrollers, pre-size rollers, straightening rollers, feed belt drivemechanism, and tube cutter;

FIG. 5 is a detailed perspective view of the pre-size and straighteningrollers of FIGS. 3 and 4, showing circumferential grooves within theindividual rollers for receiving, sizing and straightening the tubingstock;

FIG. 6 is perspective view of a typical vertical hairpin expander ofprior art, showing a ram cylinder, a pressure plate, hairpin rods andbullets used to enter and expand hairpin tubes;

FIG. 7 is a perspective view of a portion of the fixture of acontrolled-shrink rate expander of prior art, showing the bottom end ofa heat exchanger for positioning within a recessed seat in a base of thefixture plate and a piano hinge clamp that is designed to fold over theseated bottom end plate to hold it fast within the fixture base plate;

FIG. 8 is a perspective view of the fixture base plate assembly of FIG.7 showing the heat exchanger bottom end plate clamped within the fixturebase plate;

FIG. 9 is a flowchart diagram that describes the process formanufacturing heat exchangers having reduced hairpin diameters accordingto the preferred embodiment of the invention;

FIG. 10 is a detailed perspective view of pre-size rollers in a hairpinbender machine according to a preferred embodiment of the invention inwhich the pre-size rollers are set to reduce the outer diameter of stocktubing to a value below its nominal value;

FIG. 11 is a perspective view of a hairpin storage rack according to apreferred embodiment of the invention, showing individual tubularcompartments;

FIG. 12 is a perspective view of an aluminum lacing table according to apreferred embodiment of the invention having a specified table flatnessand continuous side rails;

FIG. 13 is a front elevation of a controlled-shrink-rate verticalhairpin expander according to a preferred embodiment of the invention,shown with the expander bullets at a top end of their range of motionand a bolster plate at a bottom end of its range of motion;

FIG. 14 is a front elevation of the hairpin expander of FIG. 13, shownwith the expander bullets in a low position and the bolster plate raisedby a hydraulic piston-cylinder assembly;

FIG. 15 is a perspective view of an improved bottom end plate clampassembly according to a preferred embodiment of the invention for usewith the expander of FIGS. 13 and 14 in place of the traditionalpiano-hinge clamp mechanism of FIGS. 7-8, showing a pair of bridgesmounted to a fixture base plate and a fork that slides between thebridges and the base plate for clamping the bottom end plate;

FIG. 16 is a perspective view of the improved bottom end plate clampassembly of FIG. 15, showing the fork positioned in the clampingposition, capturing the heat exchanger bottom end plate within arecessed seat of the fixture base plate;

FIG. 17 is a detailed perspective view of one of the bridges of FIGS. 15and 16; and

FIG. 18 is a perspective view of the top end of an HVAC heat exchangercondenser coil manufactured according to the preferred embodiment of theinvention, showing return bend fittings and manifolds brazed to thehairpin tubes in a single autobrazing process.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIG. 9 is a flowchart diagram that describes the process formanufacturing heat exchangers having reduced hairpin diameters accordingto the preferred embodiment of the invention.

As shown in step 50′ fins 12′, adapted for small diameter hairpins 22′(such as, but not unnecessarily limited to 5 mm tubing) are produced bya fin press in the same manner as described in step 50 with reference toFIG. 2. Details of step 50′ and of the heat exchanger fin 12′ producedthereby are disclosed in co-pending provisional U.S. patent applicationNo. 61/061,498, entitled “Method for Manufacturing Tube and Fin HeatExchanger with Reduced Tube Diameter and Optimized Fin ProducedThereby,” filed on Jun. 13, 2008, by inventors Pei Pei Chen and RussellTharp, which is incorporated herein in its entirety by reference.Likewise, in step 52′, heat exchanger top and bottom end plates 14′, 16′are formed as known in the prior art and described above with referenceto step 52 of FIG. 2, except that the end plates are dimensioned forhairpin tubes 22′ of reduced diameter.

According to the preferred embodiment of the invention, a reducedhairpin diameter (e.g. 5 mm) tube and fin heat exchanger manufacturingprocess includes a novel and unobvious processing step 200 in formingthe hairpin tubing. In the hairpin forming process of prior art as shownin FIGS. 3-5, the hairpin bender machines function by pulling rawnon-skin-hardened tubing 10 from level wound bunch coils or fromcardboard spools through a correction process that straightens thecoiled tubing and corrects dimensional abnormalities such aseccentricity or oversized diameters. The correction process is typicallycarried out by a series of cross axis rollers 102, pre-size rollers 104,and/or straightening rollers 106 before the tubing is cut and bent. Thetubing is pulled through the correction rollers by a feed belt drive 108system using two opposing feed belts. As the feed belts turn, the tubingis pulled through the hairpin production line. A tube cutter 98 cuts thestraightens tubing stock at predetermined lengths, and a tubing bender94 bends the cut tubing around a mandrel 115.

In addition to or in place of the correction process of correctingdimensional abnormalities to the nominal tubing dimensions as known inthe prior art, according to the preferred embodiment of the invention,the non-skin-hardened 5 mm tubing stock 100′ is passed through thepre-size rollers 104′, but the pre-size rollers 104′ are positioned toreduce the outer diameter of the tubing stock 100′ to a dimension thatis significantly smaller than its nominal diameter. For example, tubingstock measuring 5.05 mm is sized down to 4.86 mm (a 3.8% reduction) inpre-size rollers 104′.

FIG. 10 is a more detailed view of the pre-size rollers 104′ accordingto the preferred embodiment of the invention. Preferably, the pre-sizerollers 104′ are located prior to the drive belt system 98. The pre-sizerollers 104′ include one or more upper rollers 130 and one or more lowerrollers 132. The distance between the upper rollers 130 and the lowerrollers 132 is selectively adjustable so that the resultant outerdiameter of the tubing exiting the pre-size rollers 104′ can beprecisely adjusted and controlled. Preferably, a dial indicator or likegauge (not shown) is provided so that precise adjustments in the rollerpositions can be measured. Each individual roller 104′ includes acentral groove 105 formed within its circumference into which the 5 mmtubing stock 100′ is received. Pre-size rollers 104′ squeeze tubingstock 100′, producing tubing 100″ having an outer diameter to a valuesignificantly below nominal. This diameter reduction is the firstfeature of the manufacturing process according to the preferredembodiment of the invention that results in an improved ability to lacelong stack of fins 12′ with 5 mm or smaller hairpins 22′.

In addition to the reduction in hairpin diameter, the action of thepre-size rollers 104′ results in a skin hardening of the tubing 100″(and hence the hairpins 22′), which is a second feature of the processaccording to the preferred embodiment of the invention that results inan improved ability to lace long stacks of fins 12′ with 5 mm or smallerhairpins 22′. For example, using 5 mm copper tubing, a 2×2 fullfactorial Design of Experiments (DOE) has revealed that the cold-workingof the copper tubing outer surface by the pre-size rollers 104′ resultsin a case hardened skin-hardened tubing 100″, which in turn reducescantilevered deflection of the hairpins 22′, thus easing the lacingprocess 202. As an added benefit, the skin hardening also improves boththe yield and burst strength of the hairpins 22′, with the cost-savingresult that a pressure switch may be eliminated from various HVAC systemdesigns when using a heat exchanger manufactured according to thepreferred embodiment.

Because pre-size rollers 104′ have grooves 105 that are designed for usein returning oversized 5 mm tubing to its nominal diameter of 5.05 mm,using these pre-size rollers 104′ to reduce the tubing diameter to 4.86mm imposes a 0.05 mm ovality to the tubing. This resultant hairpinovality reduces the area of contact of hairpin tube surface with thecollared holes of the fins and is a third feature of the manufacturingprocess according to the preferred embodiment of the invention thatresults in an improved ability to lace a long stack of fins with 5 mm orsmaller hairpins 22′.

Although it is preferred to reduce the diameter of the hairpin tubesusing pre-size rollers 104′, other means for reducing hairpin diametermay be used. Additionally, other means for creating oval hairpin tubingmay be used and other means for skin hardening of tubing may be used asappropriate.

Referring to FIG. 11, once the hairpins 22′ are manufactured, they arepreferably staged in tubes 299 or other segmented racks to limit thenumber of small diameter hairpins 22′ that may be stacked one uponanother. This compartmentalization protects the hairpins 22′ from damageand maintains their dimensional integrity.

The lacing process is described in step 202 of FIG. 9, and an improvedlacing table according to a preferred embodiment of the invention isshown in FIG. 12. Lacing tables for conventional (⅜ inch heatexchangers) are typically made from welded steel. Welding results in aslight warpage of the lacing table surface. While this warpage isinsignificant for lacing ⅜ inch hairpins, the resultant misalignment offin holes makes it difficult to lace long heat exchangers with 5 mm orsmaller hairpins. According to the preferred embodiment, lacing table300 is constructed of a special grade aluminum. Table 300 includes ahorizontal surface 302 with a flatness held to within 0.001 inchesacross its width to reduce fin hole misalignment. Full length aluminumside rails 304, 306 are included to further assure fin hole alignment.

The expansion process according to the preferred embodiment is shown instep 204 of FIG. 9 and is accomplished by a novel vertical hairpinexpander 250, as described below with reference to FIGS. 13 and 14.Because hairpins 22′ have a diameter reduced below nominal, a greaterexpansion ratio must occur in order to create the required interferencefit with the collared fin holes. Thus, relative to the size of thehairpins, larger hairpin forces are encountered during expansion. If thehairpin forces are not balanced by the bolster plate to within narrowparameters, failure will result.

Expander 250 is preferably a vertical hairpin expander, built by BurrOak Tool, Inc. and modified by Crown Unlimited Machine, Inc., which isequipped to guide and assist the shrink rate of the hairpin tubes bymoving the bolster plate a controlled distance as the expansion bulletspass through the hairpin tubes. For example, a Burr Oak model CDE-M387-3vertical expander, with a tilting platform that allows horizontalloading and unloading of the heat exchangers, may be used. Expander 250includes a ram cylinder 252 that moves a pressure plate 254, which inturn drives a number of rods 256 and bullets 258 (only one is shown forsimplicity). A fixture 259 that includes a continuous front plate 260, acontinuous back plate 261 (which is disposed directly behind front plate260 in FIGS. 13 and 14), and left and right side rails 262, 264,respectively, hold the heat exchanger top and bottom end plates andstack of fins stationary within expander 250. A saddle 266 forsupporting a hairpin bend is shown connected to a receiver plate 270,which in turn is carried by a bolster plate 272. Bolster plate 272 ismoved up and down vertically by a hydraulic piston-cylinder arrangement274 (FIG. 14) that is connected between bolster plate 272 and theexpander frame 275.

Unlike the bolster plate actuators of prior art, actuator 274 iscontrolled by a closed loop control system that receives a position orforce feedback signal, thus allowing accurate, infinitely variableprogramming of the bolster plate position. The hydraulic bolster controlsystem preferably includes one or more position or force sensors 276 anda control system 278 (illustrated functionally as a labeled box) whichis connected between the hydraulic actuator system 274 and the feedbacksensor 276. Preferably, control system 278 is an electronic digitalcontrol system that uses a computer processor. As control systems arewell known in the art, control system 278 is not discussed furtherherein.

As illustrated in FIG. 13, expansion has not yet occurred, and rods 256and bullets 258 are at the top of their range of travel. Bolster plate272, receiver plate 270, and saddle 266 are at their correspondinglowest point of travel. To expand the hairpins, ram 252 is actuateddownwardly, driving pressure plate 254, rods 256 and bullets 258downwardly. Simultaneously, according to a user-programmed profile,hydraulic bolster actuators 274 are driven upwardly according to signalsgenerated by control system 278, with one or more feedback signalsrepresentative of ram 252 position or force, or bolster plate 272position or force. As hydraulic bolster actuators 274 move upwardly,they drive bolster plate 272, receiver plate 270, and saddles 266upwardly to balance the forces exerted on the hairpin tubes during theexpansion process. FIG. 14 illustrates expander 250 after expansion hasoccurred, with bullets 258 at their lowest point of travel and bolsterplate 272 at its uppermost point of travel. Because of the largeexpansion ratios (typically 8-9 percent) employed in the manufacturingprocess according to preferred embodiment of the invention, bolsterplate 272 may travel about one-tenth of the distance that bullets 258travel.

FIGS. 13 and 14 illustrate a second feature of expander 250 according toa preferred embodiment of the invention to aid in the manufacturing of 5mm or smaller heat exchangers. Hairpin expanders of prior art typicallyemploy a fixture that has a number of front plates and a number of backplates, which are repositioned to accommodate heat exchanger stacks ofvarious lengths, widths and depths. Gaps exist between adjacent frontplates and between adjacent back plates. These gaps have not beenproblematic when expanding ⅜ inch heat exchangers of prior art.Likewise, prior art expander fixtures have short, individual side railsegments that mount to front plates or back plates. Only one or 2 siderail segments are typically used per side, which act only as a guide forcentering the stack with the fixture.

However, because of the large hairpin forces involved in expanding 5 mmheat exchangers, hairpins 22′ are prone to buckle in the regions wherethere are gaps in the fixture. Therefore, as illustrated in FIGS. 13 and14, fixture 259 includes a continuous, full length, full width frontplate 260 and a continuous, full length, full width back plate 261.Likewise, fixture 259 includes full length continuous side rails 262,264. The continuous full length side rails in cooperation with thecontinuous full length front and back plates provide lateral support 360degrees around the entire stack of fins to prevent buckling. Althoughthe front and back plates 260, 261 and left and right side rails 262,264 are described as full length, in practice this means that theyextend to at least within a few inches of the heat exchanger top endplate and/or heat exchanger bottom end plate.

According to the preferred embodiment of the invention, expander 250includes a novel clamping apparatus. During the controlled-shrink-rateexpansion process, tremendous forces may be placed on the heat exchangerbottom end plate, particularly for large 5 mm heat exchangers. The largeforces are problematic, because the prior art piano hinge clamp fixture168 (FIGS. 7-8) that is typically provided with the Crown UnlimitedMachine expander is unable to hold the bottom end plate duringexpansion.

FIGS. 15 and 16 illustrate an improved clamping fixture 450 according toa preferred embodiment of the invention that replaces the prior artpiano clamp 168 of FIGS. 7-8. Clamp 450 includes two bridges 452, whichare mounted to the existing base plate 165 of a Crown Unlimited MachineInc. expander, which is part of fixture 259 (FIGS. 13-14). Bridges 452remain attached to base plate 165 regardless of whether or not a heatexchanger is clamped within expander 250. Once a heat exchanger bottomend plate 16′ is received into the recessed seat in base plate 165, aU-shaped fork member 456 is simply slid under both bridges 452 to secureheat exchanger bottom end plate 16′ in place. Fork 456 has a slot 457that accommodates the hairpin tubes 22′ of the heat exchanger. When itis desired to remove the heat exchanger, fork 456 is slid out ofengagement from under bridges 452. Two blocks 458 with an aperture 460formed in each are preferably received onto the studs 162 (see alsoFIGS. 6-7) and within slot 457 of fork 456. Clamping fixture 450provides a robust clamp required for 5 mm heat exchanger manufacture.

FIG. 17 illustrates a bridge 452 of clamp 450. Each bridge 452 mayinclude holes 454 that allow the bridge to be bolted on to base plate165.

Referring collectively to FIGS. 9 and 18, after the hairpin expansionstep 204, an autobrazing step 206 is performed according to thepreferred embodiment of the invention. In conventional ⅜ inchautobrazing, only the return bends 24 (FIG. 1) and cross-over fittingsare included. According to the preferred embodiment, in step 206,subcooler manifolds 502, liquid manifold 503, and suction manifold 506are also brazed in the autobraze process with the return bends 24′. Thisprocess step eliminates manpower variation associated with manualbrazing in the downstream assembly step 64 (FIG. 2) and reduces leaks.Step 206 has the added benefit of allowing more efficient leak testing,because all circuits can be tested simultaneously in step 62′ ratherthan having to test each fluid circuit individually, one at a time.

Due to the increased mass of copper in the elongated manifold legsversus the smaller return bend fittings, braze rings characterized by alower melting temperature are used on the manifold legs. Braze rings onreturn bends 24′ are preferably BCuP-2 equivalent, with a meltingtemperature of 1310° F. Braze rings on legs of manifolds 502, 503, 506are preferably BCu-P4 equivalent, with a melting point of 1190° F.

For example, referring to FIG. 18, a heat exchanger top end plate 14′ asmanufactured according to the preferred embodiment of the invention isshown. Upper tips 26′ of 5 mm hairpin tubes extend beyond top end plate14′. Return bend fittings 24′ are brazed to upper tips 26′ at exemplarjoints 501 using the higher temperature braze rings. The legs ofsubcooler manifold 502, liquid manifold 503, and suction manifold 506are brazed to upper tips 26′ at exemplar joints 505, 504 and 507,respectively, using the lower temperature braze rings.

Referring back to FIG. 9, at step 62′, a leak check by pressure decaymeasurement is performed in a manner similar to that of step 62 of FIG.2, except that because subcooler, liquid and suction manifolds 502, 503,506, respectively, are included in the autobrazing step 206, all of thecircuits are checked simultaneously instead of individual circuitschecked one at a time. A pressure decay tester forces dry air at highpressure into a port of a condenser coil, with the remaining ports ofthe coil sealed with quick-disconnect mechanical plugs. After a briefperiod of settle time, the pressure inside the coil is measured overtime for a pressure drop, which would be indicative of a leak.

According to a preferred embodiment of the invention, a blockage test208 is performed using the same pressure decay measurement equipmentthat is used to perform the leak check 62′. Blockage is not tested inthe manufacturing process of prior art. Smaller diameter tubing is moresusceptible to blockage from liquid braze material flowing beyond thebrazed joint and collecting in the tubing during the autobrazing process206. The pressure decay tester is connected to an input port of thecoil, but the exit port of the coil, for example, the liquid manifold,is left open, vented to atmosphere. The pressure decay tester injectshigh pressure air into the coil, and the pressure sensed by the pressuredecay tester is measured. A normally unblocked coil will maintain aninternal pressure in this arrangement due to natural restrictions in thecoil from return bends, changes in tubing diameter, and the long lengthof the small diameter circuit. For example, a particular, unblocked 5 mmcoil subjected to a charge of 325 psi dry air will maintain an internalpressure of 190-195 psi. If there is a blockage or partial blockage inthe coil, the sensed pressure will be at an increased level.

The Abstract of the disclosure is written solely for providing theUnited States Patent and Trademark Office and the public at large with away by which to determine quickly from a cursory reading the nature andgist of the technical disclosure, and it represents solely a preferredembodiment and is not indicative of the nature of the invention as awhole.

While some embodiments of the invention have been illustrated in detail,the invention is not limited to the embodiments shown; modifications andadaptations of the above embodiment may occur to those skilled in theart. Such modifications and adaptations are in the spirit and scope ofthe invention as set forth herein:

1. A method for manufacturing a tube and fin heat exchanger comprisingthe steps of: providing a supply of non-skin-hardened tubing having anominal outer diameter; and reducing the outer diameter of saidnon-skin-hardened tubing to a value significantly smaller than saidnominal outer diameter so as to produce a supply of skin-hardened tubingfrom said non-skin-hardened tubing; wherein said step of reducing theouter diameter further comprises the step of rolling saidnon-skin-hardened tubing between first and second pre-size rollers;whereby said step of rolling creates an ovality in said skin-hardenedtubing; cutting said skin-hardened tubing to plurality of predeterminedlengths; bending said plurality of predetermined lengths of tubing 180degrees into hairpin tubes; lacing said plurality of hairpin tubesthrough a plurality of apertures in a stack of fins sandwiched betweenfirst and second endplates; and expanding the outer diameter of saidlaced hairpin tubes to create an interference fit between said hairpintubes and said plurality of apertures.
 2. The method of claim 1 whereinsaid step of expanding the outer diameter further comprises the stepsof: holding said first and second end plates in a fixture; supportingsaid hairpin tubes at their bent ends by a bolster plate; expanding saidhairpin tubes by a plurality of bullets driven by a ram actuator; movingsaid bolster plate by a hydraulic actuator to compensate for hairpincontraction caused by said step of expanding; and controlling saidhydraulic actuator with a closed loop control system.
 3. The method ofclaim 2 further comprising the steps of: controlling said hydraulicactuator as a function of force on said hairpin tubes.
 4. The method ofclaim 2 further comprising the step of: controlling said hydraulicactuator as a function of the position of said ram actuator.
 5. Themethod of claim 2 further comprising the step of: controlling saidhydraulic actuator as a function of the position of said bolster plate.6. The method of claim 1 further comprising the steps of: brazing areturn bend fitting to a first of said plurality of hairpin tubes; andsimultaneously brazing a manifold to a second of said plurality ofhairpin tubes.
 7. The method of claim 6 further comprising the steps of:brazing said return bend fitting to said first of said plurality hairpintubes using a first braze ring characterized by having a first meltingtemperature; and simultaneously brazing said manifold to said second ofsaid plurality of hairpin tubes using a second braze ring characterizedby a second melting temperature lower than said first meltingtemperature.
 8. The method of claim 1 further comprising the steps of:expanding said hairpin tubes by an expansion ratio greater than 6percent.
 9. The method of claim 1 further comprising the steps of:expanding said hairpin tubes by an expansion ratio greater than 8percent.
 10. The method of claim 1 further comprising the steps of:venting a first end of a fluid circuit of said heat exchanger toatmosphere; pressurizing a second end of said fluid circuit; andmeasuring the pressure in said fluid circuit to verify said fluidcircuit has no blockage.
 11. The method of claim 1 further comprisingthe steps of: reducing the outer diameter of said non-skin-hardenedtubing by 3.8 percent so as to produce said skin-hardened tubing fromsaid non-skin-hardened tubing.
 12. A method for manufacturing a tube andfin heat exchanger comprising the steps of: providing a supply ofskin-hardened tubing; cutting said tubing to plurality of predeterminedlengths; bending said plurality of predetermined lengths of tubing 180degrees into hairpin tubes; lacing said plurality of hairpin tubesthrough a plurality of apertures in a stack of fins sandwiched betweenfirst and second endplates; expanding the outer diameter of said lacedhairpin tubes to create an interference fit between said hairpin tubesand said plurality of apertures; brazing a return bend fitting to afirst of said plurality hairpin tubes using a first braze ringcharacterized by having a first melting temperature; and simultaneouslybrazing a manifold to a second of said plurality of hairpin tubes usinga second braze ring characterized by a second melting temperature lowerthan said first melting temperature.